The recent success of imatinib for the treatment of Philadelphia chromosome positive chronic myelogenous leukemia has made tyrosine and serine/threonine kinases major targets for cancer therapy. As only a small fraction of the human kinome can currently be targeted by a reasonably selective and potent inhibitor, there is an urgent need to develop strategies for efficient discovery and optimization of new inhibitors. Towards this goal, we have recently developed a compound library of ATP-competitive kinase inhibitors. Using our collection of novel compounds (approximately 2,000), we tested a panel of 16 cultured tumor cell lines for the ability of these compounds to induce apoptotic death. This search revealed a compound, ON1231320, that had remarkable cytotoxicity against the entire panel of 16 tumor cell lines, with little or no cytotoxicity towards normal cells. Kinase inhibition assays against a panel of 285 kinases revealed that this compound had a remarkable specificity towards Plk2/Snk, a kinase involved in centrosome duplication and mitotic progression. Most importantly, Plk2 has recently been implicated as one of the kinases that links cellular metabolism to cell cycle. Mitochondrial dysfunction with increased dependence on glycolysis is frequently observed in cancer cells (known as the Warburg effect) and identification of pathways that promote cell survival under conditions of mitochondrial dysfunction have therapeutic implications. In a recent study, it has been shown that targeted ablation of SCO2 gene in HCT116 human colon cancer cell line results in the ablation of mitochondrial respiration and that PLK2 is the most highly expressed gene in SCO2-/-cells. Even a modest reduction in PLK2 levels in human cancer cells with defects in mitochondrial respiration results in the elimination of their ability to form xenografts in mice. Our results show that ON1231320 is a potent inducer of tumor cell death and has an excellent safety profile in vivo. We propose to study the effects of this compound on tumor growth in vitro and in vivo to determine how ON1231320 will serve as a novel cancer chemotherapeutic. The aims of the proposal are: 1. To determine the kinetics of inhibition of Plk2 by ON01231320; 2. To determine the effects of ON1231320 on downstream signaling events mediated by Plk2 and evaluate its effect on tumor cell growth under hypoxic, genotoxic and defective respiratory contexts; 3. Characterize the PK properties of ON1231320 in xenograft models to determine the degree of inhibition of Plk2 that is required for inhibition of tumor growth and assess how ON1231320 concentrations are related to antitumor activity of the compound and to evaluate the validity of using PBMN as surrogates; and 4. To further explore the effects of combination therapy using ON1231320 and cytotoxic anti-cancer agents such as oxaliplatin, 5-fluorouracil, paclitaxel and doxorubicin to induce apoptosis and tumor regression in breast and colorectal cancer model systems.